(a) Field of the Invention
The invention relates to development of agonist or antagonist of a G protein-coupled receptor, which bind to the G protein-coupled receptor from the extra-cellular side in a manner different from that of the natural ligand.
(b) Description of Prior Art
Prostaglandins are derived from the oxygenation of arachidonic acid by prostaglandin synthetases. Prostaglandins mediate a wide variety of physiological actions, such as vasomotricity, sleep/wake cycle, intestinal secretion, lipolysis, glomelular filtration, mast cell degranulation, neurotransmission, platelet aggregation, leuteolysis, myometrial contraction and labor, inflammation and arthritis, patent ductus arteriosus, cell growth and differentiation (Coleman, R. A. et al., 1994, Pharmacol. Rev. 46:205–229; Goetzl, E. J. et al., 1995, FASEB J. 9:1051–10585). Prostanoids mediate their actions through binding to distinct receptors, which belong to the super family of rhodopsin-like seven transmembrane helical receptors. These receptors are coupled to heterotrimeric G-proteins comprising of α, β and γ subunits which, upon activation, elicit alterations in cell calcium, initiate phosphoinositide hydrolysis or promotion or repression of cyclic adenosine monophosphate synthesis (Strader C. D. et al., 1994, Ann. Rev. Biochem. 63:101–132).
Of the five pharmacologically distinct prostanoid receptors for PGE2, PGI2, PGD2, PGF2α and TxA2 and their many isoforms, the receptor for PGF2α, also called FP receptor, shows limited tissue distribution, predominantly expressed in corpora leutea, uterine myometrium, trabecular meshwork of the eye, and to a lesser extent in vascular smooth muscle. Initiation of labor is marked by tremendous rise in PGF2α, levels and increased uterine contractility. The wide spread use of PGF2α analogues to induce labor in veterinary industry points to the primary role of PGF2α and its receptor in parturition. This is underscored by the fact that mice lacking the FP receptor fail to undergo labor (Sugimoto et al., 1997, Science 277:81–83). In face of escalating costs incurred as a result of premature births and associated complications to the neonate, such as intraventricular hemorrhage, bronchopulmonary displasia and periventricular leukomalacia leading to cerebral palsy, prolongation of gestation by arresting premature labor is an effective preventive therapy. The relative success of nonsteroidal anti-inflammatory drugs as a short-term therapy toward prevention of premature labor is based on their inhibitory actions upon the synthesis of prostaglandins, particularly PGE2 and PGF2α. However, inhibition of PGE2 is associated with serious complications to the fetus such as the closure of ductus arteriosus, renal failure and pulmonary hypertension.
At another level, PGF2α, has been attributed a major role in dysmenorrhea, a condition which afflicts 5%–7% of premenopausal women. A pre-menstrual increase in PGF2α levels resulting in myometrial spasms underlies the pathogenesis of this disorder. Lack of effective antagonists of FP receptor for extended therapy hampered the advances in preventing premature labor and associated sequelae, and the design of such antagonists is the subject of this application.
Human FP receptor is a 45 kDa integral membrane glycoprotein, consisting of 359 amino acids and shares only 47% sequence identity with EP1 receptor, and to a lesser extent with other prostanoid receptors (Abramovitz et al., 1994, J. Biol. Chem. 269:2632–2636). Binding of PGF2α to FP receptor is followed by the activation of Gαqβγ complex, increased GTP binding by the Gαq subunit, stimulation of phospholipase Cβ activity, release of inositol phosphates, increased intracellular calcium and subsequent signal transduction phenomena ultimately leading to smooth muscle contraction (Coleman, R. A. et al., 1994, Pharmacol. Rev. 46:205–229). The FP receptor is the only efficacious target for development of therapeutic drugs since a few Gα-proteins catalyze the actions of hundreds of G-protein coupled receptors, thus targets downstream from the receptor are essentially of little use.
Antagonists of FP receptors directed to the ligand binding site could be of limited use since ligand based inhibitors show cross reactivity with other prostanoid receptors. Their efficacy will be compromised in face of tremendous increase in PGF2α, concentrations in myometrium at the onset of labor and in menstruation. The high basal activity of the receptors in the absence of ligand limits the use of ligand-based inhibitors.
It would be highly desirable to be provided with agonist or antagonist of FP receptors, which do not crossreact with other prostanoid receptors, and are effective even in the presence of excess ligand.